With the proliferation of wireless communications standards and frequency bands, future mobile terminals will often support several frequency bands as well as several cellular system standards. One cost- and size-effective approach for a mobile terminal (a User Equipment, or UE, in the parlance of the 3rd-Generation Partnership Project, or 3GPP) to support a number of frequency bands is to permit only half-duplex operation in one or more of the frequency bands. Half-duplex operation means that the mobile terminal does not support simultaneous transmission and reception, meaning that a large duplex filter is not needed. Duplex filters generally also cause a significant signal power loss; hence, half-duplex operation also provides benefits in terms of mobile terminal power consumption, especially at high output powers.
FIG. 1 shows an example of half-duplex operation in a system that supports both full-duplex operation, such as 3GPP's Long Term Evolution (LTE) system. With the pictured allocation of uplink and downlink resources, the scheduled mobile terminal operates in half-duplex operation (i.e., no simultaneous reception and transmission) even though the uplink and downlink resources are in distinct frequency bands. In this example a mobile terminal is scheduled to receive data in two downlink subframes 120, i.e., the subframes numbered “0” and “1”, and to transmit data in two uplink subframes 110, i.e., the subframes numbered “3” and “4.” In the pictured example, the allocation of resources in the first frame 100 (comprising ten uplink subframes 120 and ten downlink subframes 110) is repeated in the subsequent frame. The scheduled uplink and downlink subframes do not overlap; thus, the scheduled mobile terminal can operate in half-duplex mode, using a duplexing switch rather than an expensive and bulky duplex filter.
Although half-duplex operation provides advantages in cost and size, one drawback is that maximum allowed throughput is reduced, because fewer than all subframes may be allocated to the uplink and/or the downlink at a given time. However, for LTE and future cellular systems supporting data rates up to and above 100 MB/s, half-duplex terminals may still reach high data rates (e.g., greater than 20 Mb/s).
As noted above, duplex filters introduce front-end power loss and may increase the power consumption of a mobile terminal. However, duplex filters also provide some benefits other than simply allowing simultaneous transmission (TX) and reception (RX). A primary effect of the filter is to reduce the transmitter power that leaks into the mobile terminal's receiver. Without the filter, the energy from the transmitter is likely to desensitize a simultaneously operating receiver circuit. However, the filter also reduces terminal-to-terminal interference, especially in systems using frequency plans in which the frequency duplex distance (i.e., the separation between transmit and receive frequencies) is small relative to the system bandwidth. One example is the 700 MHz band, currently planned for use in future cellular systems, especially in the United States. In the 700 MHz band, the system bandwidth is around 5 MHz, with a duplex distance of only 10-15 MHz. Especially in these systems, the transmitter noise power in the receive band can be at a high level before it is attenuated by a duplex filter.
FIG. 2 illustrates a scenario in which the benefits of having a duplex filter are demonstrated. As shown in FIG. 2A, mobile terminals 220 and 230, designated MT1 and MT2, respectively, are situated close to one another (e.g., one meter apart) and are communicating with a distant base station 210. Thus, the output power transmitted by mobile terminal MT1 is relatively high (e.g., 20 dBm), as shown in FIG. 2B, while the signal received at mobile terminal MT2 from the base station is relatively low (e.g., −90 dBm), as shown in FIG. 2D. In this case, mobile terminal MT1 is transmitting at the same time that mobile terminal MT2 is receiving.
As shown in FIG. 2B, the output power spectrum from transmitting mobile terminal MT1 is at its peak at the designated transmit frequency fTX. Although the power spectral density rolls off outside of the transmit frequency band, considerable transmit energy is still present in the receive frequency band, centered at fRX. In FIG. 2B, the transmit power in the receive band (prior to filtering, if any) is −20 dBm, 40 dB below the transmit band power level. As shown in FIG. 2C, the duplex filter in mobile terminal MT1 suppresses the out-of-band emissions resulting from mobile terminal MT1's transmission; in the pictured example, the filter provides 45 dB of rejection for receive-band emissions from the transmitter circuitry. Thus, the transmitted power in the receive band is at approximately −65 dBm.
In the scenario pictured in FIG. 2A, the coupling loss between the two mobile terminals 220 and 230 might be as low as 40 dB, for example. In this case, then, the noise floor at the input of the receiver for mobile terminal MT2, resulting from receive-band emissions from mobile terminal MT1, is 20 dBm−40 dB−45 dB=−105 dBm, well below the received signal level for the desired signal from base station 210 (at −90 dBm). This is illustrated in FIG. 2D.
FIGS. 3A-3C illustrate a similar scenario, except that FIG. 3A depicts two half-duplex mobile terminals 320 and 330, respectively designated MT3 and MT4. Unlike the mobile terminals in FIG. 2, these mobile terminals have no duplex filters. In this case, the lack of a duplex filter makes the noise floor 45 dB higher than in the scenario pictured in FIG. 2, implying, given the same conditions discussed above, a noise floor at the receiver of mobile terminal MT4 of around −60 dBm, as pictured in FIG. 3C. Because this is 30 dB higher than the received signal level of −90 dBm, mobile terminal MT4 will not be able to receive and decode the signal from the base station 210. If the interference persists, mobile terminal MT4 will eventually lose synchronization with base station 210.
Those skilled in the art will appreciate that the coupling loss is increased if mobile terminals MT3 and MT4 are separated by a larger distance, thus reducing the interference. However, the coupling loss increases quite slowly, i.e., by about 12 dB for each doubling of the distance. Hence, increasing the distance to four meters results in a coupling loss approximately 24 dB higher. However, at this distance the noise floor is at −84 dBm for the conditions described above, still well above the received signal level.